Scanning optical system and image forming apparatus

ABSTRACT

A system includes a light source, a deflection unit configured to deflect a light beam having a wavelength λ emitted from the light source, and a lens unit including a plurality of lenses that focuses deflected light on a surface to be scanned, at least one lens among the plurality of lenses has a micro concavo-convex structure in an optical surface, and the optical surface having the micro concavo-convex structure has a transmittance distribution for the light beam having the wavelength λ according to a light quantity distribution of the deflected light and entering the lens unit.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

The aspect of the embodiments relates to a scanning optical system, anoptical scanning device, and an image forming apparatus that use anoptical element having a transmittance distribution in a light scanningdirection.

Description of the Related Art

Conventionally, a scanning optical system such as a laser beam printer(LBP) periodically deflects, using an optical deflector, a light beamemitted from a light source that is optically modulated according to animage signal, focuses the light beam in a spot manner on a recordingmedium surface using an image forming optical system, and scans therecording medium to record an image.

FIG. 8 illustrates a cross-sectional view of main part in amain-scanning direction (main-scanning cross-sectional view) of aconventional scanning optical system. In FIG. 8, a scanning opticalsystem 80 includes a light source 81, and the light source 81 includes,for example, a semiconductor laser. Light emitted from the light source81 passes through a collimator lens 82 and is converted into asubstantially parallel light beam, and an aperture stop 83 limits lightbeam that passes therethrough to shape a beam form. An optical deflector84 includes, for example, a four-sided polygon mirror (rotary polygonmirror), and rotates at a constant speed in a direction of arrow A inFIG. 8 by a driving unit (not illustrated) such as a motor and deflectslight.

The light beam based on image information, that has been reflected anddeflected by the optical deflector 84 is focused on a photosensitivedrum surface 88 as a surface to be scanned by a first scanning lens 86and a second scanning lens 87, which constitute a scanning optical lenssystem 85 having a light-condensing function and an f0 characteristic.

Such a conventional scanning optical system has an issue of occurrenceof light quantity unevenness where the light quantity in themain-scanning direction on the surface to be scanned by the light beambecomes uneven due to reflection characteristics of a reflection surfaceof the rotary polygonal mirror. The light quantity unevenness causesdeterioration of quality of an image recorded on the recording mediumsurface of the LBP.

Japanese Patent Application Laid-Open No. 2011-154115 discusses, toresolve the issue, a scanning optical system that uniformizes a lightquantity on a surface to be scanned by including a light quantitycorrection optical film having a transmittance distribution in amain-scanning direction on an optical path of the scanning opticalsystem. Specifically, Japanese Patent Application Laid-Open No.2011-154115 discusses the light quantity correction optical filmobtained by forming a film of a light shielding material on a lighttransmitting member by a deposition method.

SUMMARY OF THE DISCLOSURE

The aspect of the embodiments relates to a system including a lightsource, a deflection unit configured to deflect a light beam having awavelength λ emitted from the light source, and a plurality of lensesthat focuses deflected light on a surface to be scanned, in which atleast one lens among the plurality of lenses has a micro concavo-convexstructure in a surface, and a lens surface having the microconcavo-convex structure in the surface has a larger transmittance at alens center than a transmittance at a lens end portion for the lightbeam having the wavelength λ.

Further, the aspect of the embodiments relates to an apparatus includinga scanning device including the above-described system, a photosensitivedrum disposed on a surface to be scanned of the optical scanning device,a developing unit configured to develop, as a toner image, anelectrostatic latent image formed by a light beam scanning thephotosensitive drum, a transfer unit configured to transfer thedeveloped toner image on a sheet, and a fixing unit configured to fixthe transferred toner image on the sheet.

Further features of the disclosure will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view in a main-scanning direction of ascanning optical system according to the present exemplary embodiment.

FIG. 2 is a diagram illustrating incident light dependency of a lightbeam reflectance in an optical deflector.

FIG. 3A is a schematic view of a lens and FIG. 3B is a schematic view ofa micro concavo-convex structure according to the present exemplaryembodiment.

FIGS. 4A and 4B are graphs illustrating correlations between dimensionsof the micro concavo-convex structure and transmittance.

FIG. 5 is a schematic view of an image forming apparatus according tothe present exemplary embodiment.

FIGS. 6A to 6G are cross-sectional views of a process of manufacturingthe micro concavo-convex structure according to the present exemplaryembodiment.

FIGS. 7A to 7G are cross-sectional views of a process of manufacturing amicro concavo-convex structure according to a third exemplaryembodiment.

FIG. 8 is a cross-sectional view in a main-scanning direction of ascanning optical system according to a conventional technology.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, exemplary embodiments of the disclosure will be describedin detail with reference to the drawings.

(Scanning Optical System)

FIG. 1 is a cross-sectional view in a main-scanning direction of ascanning optical system according to the present exemplary embodiment.In FIG. 1, a scanning optical system 10 includes a light source 11, and,for example, a semiconductor laser can be used as the light source 11.Light (wavelength λ) emitted from the light source 11 is deflected by anoptical deflector (deflection unit) 12. In the present specification, adirection in which a light beam is deflected by the optical deflector 12is defined as the main-scanning direction. The optical deflector 12includes, for example, a four-sided polygon mirror, and rotates at aconstant speed in a direction of arrow A in FIG. 1 using a driving unit(not illustrated) such as a motor.

A scanning lens system 13 as a scanning optical unit having alight-condensing function and fθ characteristics is a lens unitincluding a plurality of lenses made of a resin material. In FIG. 1, thescanning lens system 13 includes a first scanning lens 14 and a secondscanning lens 15. The first scanning lens 14 has an optical surface 14 aon the optical deflector 12 side and an optical surface 14 b oppositethe optical surface 14 a. Similarly, the second scanning lens 15 has anoptical surface 15 a on the optical deflector 12 side and an opticalsurface 15 b opposite the optical surface 15 a. The scanning opticalsystem 10 according to the present exemplary embodiment has a microconcavo-convex structure in an optical surface of at least one of thefirst scanning lens 14 and the second scanning lens 15. The scanningoptical system 10 according to the exemplary embodiment favorably hasthe micro concavo-convex structure in at least two or more opticalsurfaces of a plurality of lenses. Here, the optical surface refers to asurface that the light beam is assumed to pass through when the lens isdesigned.

In the scanning optical system 10, in one embodiment, that the firstscanning lens 14 and/or the second scanning lens 15 has the microconcavo-convex structure in both optical surfaces that are opposite eachother. Further, it is more favorable that the first scanning lens 14 andthe second scanning lens 15 have the micro concavo-convex structure inthe both optical surfaces 14 a and 14 b and 15 a and 15 b, which areopposite each other.

It is favorable that the first scanning lens 14 has the microconcavo-convex structure in the two optical surfaces 14 a and 14 b andthe second scanning lens 15 has the micro concavo-convex structure inthe two optical surfaces 15 a and 15 b.

The light beam reflected and deflected by the optical deflector 12passes through the scanning lens system 13 and focuses an image on asurface to be scanned 16.

FIG. 2 illustrates an example of incident light dependency of a lightbeam reflectance in the optical deflector 12. Here, an incident angle θis an angle formed by an incident direction of the light beam incidenton a reflection surface of the optical deflector and a normal line tothe reflection surface. For example, in a case where a range of theincident angle θ to be used is 22° to 68°, the light quantity of thereflected and deflected light beam has a distribution of approximately1.9% in the main-scanning direction.

Each lens surface of the first scanning lens 14 and the second scanninglens 15 in the present exemplary embodiment has a spherical oraspherical shape in the main-scanning cross-section as illustrated inFIG. 1. The optical system according to the exemplary embodiment has themicro concavo-convex structure in at least one surface of the firstscanning lens 14 or the second scanning lens 15. The microconcavo-convex structure formed in the scanning lens controls thereflectance (transmittance) of the light beam reflected and deflected bythe optical deflector 12 to uniformize the light quantity on the surfaceto be scanned 16.

FIG. 3A illustrates an example of the scanning lens according to thepresent exemplary embodiment. In FIG. 3A, the micro concavo-convexstructure is provided in one surface 14 a of the first scanning lens 14.In FIG. 3A, the first scanning lens 14 has a lens center 31 in a centerC of an optically effective region and a lens end portion 32 in an endin the optically effective region in a direction B in which a scan isperformed using light (hereinafter the direction B is called scanningdirection). Here, the lens center 31 refers to a region with a width of1 cm including the center C (the center in the scanning direction of thelens) and the lens end portion 32 refers to a region with a width of 1cm from the end of the optically effective region.

The first scanning lens 14 of the present exemplary embodiment has thetransmittance distribution for the light beam having the wavelength λsuch that variation in the light quantity of scanning light afterpassing through the scanning lens system 13 becomes small.

For example, a case of performing a scan with the light beam in a rangeW of the incident angle θ of 22° to 68° using the optical deflector 12illustrated in FIG. 2 will be described. As can be seen from FIG. 2, thelight beam having the incident angle of approximately 45° for enteringthe lens center 31 has a lower reflectance at the optical deflector 12and thus has a smaller light quantity than the light beams having theincident angles of approximately 22° and 68° for entering the lens endportion 32. Therefore, the micro concavo-convex structure is provided inthe surface 14 a such that the transmittance of the first scanning lens14 with respect to the light having the wavelength λ becomes larger atthe lens center 31 than at the lens end portion 32. Thereby, the lightquantity of the light beam in the range of the incident angle θ of 22°to 68° after passing through the scanning lens system 13 can be uniform.That is, the transmittance of the scanning lens system 13 has adistribution according to a light quantity distribution of the lightdeflected by the optical deflector 12 and entering the scanning lenssystem 13. Hereinafter, the micro concavo-convex structure may be simplydescribed as microstructure.

In the first scanning lens 14, the transmittance of the light beamhaving the wavelength λ favorably monotonically decreases from the lenscenter 31 toward the lens end portion 32.

FIG. 3B is a schematic view of the micro concavo-convex structure,(vertically inverting and) enlarging a region illustrated by the brokenline in FIG. 3A. In the case of the scanning lens 14 illustrated in FIG.3, the transmittance is controlled by the microstructure includingtriangularly arrayed cylindrical holes, so that the light quantity inthe main-scanning direction is adjusted and the light quantity on thesurface to be scanned 16 is uniformized. The micro concavo-convexstructure is not limited to the cylindrical hole, and may be a conicalhole, a prismatic hole, or a pyramidal hole. Further, the microconcavo-convex structure may be a structure having micro protrusions ofcylinders, cones, prisms, or pyramids, or instead, a line-and-spacepattern or the like can be used. Note that the microstructure can haveany pitch as long as no diffraction occurs under use conditions.However, if the pitch is too small, production becomes difficult.Therefore, in practice, the pitch is favorably 100 nm to 900 nm, bothinclusive, and is more favorably 300 nm to 500 nm, both inclusive. Ahole depth is favorably 100 nm to 220 nm, both inclusive, more favorably110 nm to 210 nm, both inclusive, and further more favorably 125 nm to195 nm, both inclusive.

In the case of the columnar hole structure as illustrated in FIG. 3, themicrostructure can be expressed in terms of a hole pitch P, a hole depthD, and a porosity V. The porosity V corresponds to the cubic content ofholes per unit cubic content. In the pitch range where diffracted lightis not generated, the transmittance can be controlled by adjusting oneor both of the hole depth D and the porosity V.

First, the pitch P will be described. To prevent diffraction in the air,the following expression (1) is to be satisfied, where λ is thewavelength of the light beam, N₀[λ] is a refractive index of air at thewavelength of the light beam, and N₁[λ] is a refractive index of thescanning lens at the wavelength of the light beam.

P<λ/(N ₀[λ]+N ₀[λ]×sin α)   (1)

Next, to prevent diffraction inside the scanning lens, the followingexpression (2) is to be satisfied.

P<λ/(N ₁[λ]+N ₁[λ]×sin α)   (2)

Next, the hole depth D and the porosity V will be described. λ is thewavelength of the light beam, N[λ] is an equivalent refractive index ofthe microstructure at the wavelength of light beam, N₀[λ] is arefractive index of air at the wavelength of the light beam, and N₁[λ]is a refractive index of the scanning lens at the wavelength of thelight beam. The equivalent refractive index N of the microconcavo-convex structure can be obtained by the following expression (3)using the porosity V.

N[λ]=N ₀[λ]×V+N ₁[λ]×(1−V)   (3)

The microstructure can be treated as a single layer film having a filmthickness of the hole depth D and the refractive index N[λ] as long asthe pitch P is set in the range where no diffracted light occurs.Therefore, the refractive index of the scanning lens, that is, thetransmittance can be designed by performing general optical calculationusing the hole depth D and the porosity V.

FIGS. 4A and 4B are examples of graphs illustrating correlations betweendimensions of the microstructure illustrated in FIG. 3 andtransmittance. Here, the pitch P is set in the range where diffractedlight does not occur, and the wavelength of the light beam was 790 nm,which is generally used as light source unit. FIG. 4A is a correlationgraph of the hole depth D and the transmittance at the porosity V of55%. Further, FIG. 4B is a correlation graph of the porosity V and thetransmittance at the depth D of 160 nm. As can be seen from thesegraphs, the transmittance of the scanning lens can be controlled usingthe hole depth (the difference in heights of a top of a convex and abottom of a concave) D and the porosity V.

By providing the dimensional distribution of the microstructure in themain-scanning direction of the scanning lens system, the transmittancedistribution according to the light quantity of the light beam deflectedby the optical deflector and entering the scanning lens system 13 isconfigured, and the light quantity on the surface to be scanned 16 canbe uniformized.

While the micro concavo-convex structure has the role of adjusting thetransmittance of the scanning lens system 13, the micro concavo-convexstructure also has an effect of preventing light reflection occurring onthe surface of the surface 14 a at the same time. As a result, straylight caused by reflection on the surface of the surface 14 a can bereduced, and occurrence of uneven light quantity due to the stray lightcan be suppressed. An example of providing the micro concavo-convexstructure in one surface 14 a of the first scanning lens 14 has beendescribed. However, the micro concavo-convex structure may be providedin at least one of the surfaces 14 a and 14 b of the first scanning lens14 and the surfaces 15 a and 15 b of the second scanning lens 15 throughwhich the light beam passes. Moreover, the micro concavo-convexstructure may be provided in arbitrary two or three or all of thesurfaces 14 a, 14 b, 15 a, and 15 b. The dimensional distribution of themicrostructure per surface can be made smaller as the number of surfaceswhere the micro concavo-convex structure is formed is larger.

(Electrophotographic Device)

An image forming apparatus using the scanning optical system accordingto the aspect of the embodiments as a laser optical system of a copieror a multifunction machine will be described. FIG. 5 is a schematic viewof a copier. A copier 51 includes an image reading unit 52 and an imageforming unit 53. The image forming unit 53 includes a laser opticalsystem 50.

The image forming unit 53 further includes a photosensitive drum 54disposed on a surface to be scanned of a laser optical system 50, and adeveloping unit 55 that develops, as a toner image, an electrostaticlatent image formed on the photosensitive drum by scanning thephotosensitive drum with a light beam. The image forming unit 53includes a transfer unit 56 that transfers the developed toner image ona sheet P, and a fixing unit 57 that fixes the transferred toner imageon the sheet.

By using the laser optical system according to the aspect of theembodiments for the laser optical system 50, the light quantity used toscan the photosensitive drum can be uniformized and an image with highimage quality can be formed. Further, as described above, the microconcavo-convex structure has the effect of adjusting the transmittanceof the scanning lens system 13 and preventing reflection at the sametime. Therefore, the stray light caused by the reflection on the surfacecan be reduced. Therefore, the uneven light quantity (so-called ghost)caused by the stray light can be suppressed, and an image with higherimage quality can be realized.

(Method of Manufacturing Scanning Lens)

A method of forming the microstructure in the scanning lens of thepresent exemplary embodiment will be described with reference to FIGS.6A to 6G.

As illustrated in FIG. 6A, an injection molding mold 61 for molding thescanning lens is prepared. The injection molding mold 61 includes astainless steel base 61 a and a nickel alloy mirror surface 61 b.

As illustrated in FIG. 6B, a photoresist layer 62 is formed by spincoating. Spin coating can be performed at 500 rpm to 4000 rpm, bothinclusive, for 10 seconds to 60 seconds, both inclusive. The filmthickness of the photoresist layer is favorably 100 nm to 10000 nm, bothinclusive, and more favorably 100 nm to 500 nm, both inclusive.

As illustrated in FIG. 6C, the photoresist layer 62 is exposed by aninterference exposure method and is then developed, so that aphotoresist pattern 63 can be obtained. The interference exposure can beperformed with laser light having a wavelength of 254 to 365 nm, bothinclusive, for an exposure time of 0.1 to 10 seconds, both inclusive.

As illustrated in FIG. 6D, a nickel alloy pattern 64 is grown in arecess in the photoresist pattern 63 by a plating method.

As illustrated in FIG. 6E, the height of the nickel alloy pattern 64 isadjusted by an ion milling method using Ar ions. The milling time ofeach area is controlled so as to monotonically increase the height fromthe center 31 to the end portion 32.

As illustrated in FIG. 6F, the photoresist pattern 63 is removed byultrasonic cleaning using a peeling liquid, and a microstructure mold 65having an inverted structure of the microstructure formed in thescanning lens surface is obtained.

Next, as illustrated in FIG. 6G, by performing injection molding usingthe microstructure mold 65, the microstructure can be transferred to thesurface at the same time with formation of the scanning lens. Thescanning lenses 14 and 15 having the microstructure in the surfaces areobtained in this manner

Note that a surface where the microstructure is not formed is formedusing a mold having a nickel alloy mirror surface on a stainless base.

The following exemplary embodiments and comparative examples wereevaluated by the following method.

(Method of Measuring Transmittance)

The transmittance was measured using a spectroscope device (V-7300DS,manufactured by JASCO Corporation). Since transmitted light is refractedby a lens to be measured, evaluation was conducted using an integratingsphere. The transmittances at the lens center 31 and the lens endportion 32 were evaluated in arbitrary φ1-mm regions included in therespective regions.

(Method of Measuring Reflectance)

The reflectance was measured using a micro-spectroscope (LVmicro,manufactured by Lambda Vision Inc.). The measurement wavelength was 380nm to 1600 nm, and evaluation was performed using the reflectance valueof 790 nm. The reflectances at the lens center 31 and the lens endportion 32 were evaluated in arbitrary φ1-mm regions included in therespective regions.

(Evaluation of Light Quantity Distribution on Surface to be Scanned)

The light quantity distribution was measured by arranging memberssimilarly to the scanning optical system designed for laser beam printer(LBP) products and installing a light quantity sensor at the position ofthe surface to be scanned.

In a first exemplary embodiment, the micro concavo-convex structure andthe transmittance distribution were provided in the optical surface 14 aon the optical deflector 12 side of the first scanning lens 14 in thescanning optical system in FIG. 1. The optical deflector 12 has theincident light dependency illustrated in FIG. 2, and the range of theincident angle θ to be used is 22° to 68°. Note that the other surfaceis a mirror surface and is not provided with the transmittancedistribution.

First, the microstructure was provided in the optical surface of thefirst scanning lens 14 by the following method.

The method of forming the microstructure in the first exemplaryembodiment of the disclosure will be described with reference to FIGS.6A to 6G. FIGS. 6A to 6G are cross-sectional views of a process of thefirst exemplary embodiment.

First, as illustrated in FIG. 6A, the injection molding mold 61 formolding the scanning lens was prepared. The injection molding mold 61including the stainless steel base 61 a and the nickel alloy mirrorsurface 61 b was used.

As illustrated in FIG. 6B, the photoresist layer 62 was formed by thespin coating. Spin coating conditions were 1000 rpm/20 seconds, and thefilm thickness of the photoresist layer 62 was approximately 350 nm.

Next, as illustrated in FIG. 6C, the photoresist layer 62 was exposed bythe interference exposure method and was then developed, so that thephotoresist pattern 63 was obtained. In the first exemplary embodiment,a hole pattern having a triangular lattice array was formed with a cycleof 400 nm. Further, the diameter of the hole was approximately 312 nm,and the depth was approximately 350 nm, which is equivalent to the filmthickness of the photoresist layer 62.

Next, as illustrated in FIG. 6D, the nickel alloy pattern 64 was grownin the recess in the photoresist pattern 63 by the plating method. Theplating time was adjusted such that the height of the nickel alloypattern 64 became approximately 310 nm.

Next, the ion milling method using Ar ions was performed in an area unitof several 100 μm to adjust the height of the nickel alloy pattern 64 asillustrated in FIG. 6E. In the first exemplary embodiment, the millingtime was adjusted to set the height of the center 31 to approximately160 nm and the height of the end portion 32 to approximately 243 nm.Further, the milling time of each area was controlled so as tomonotonically increase the height from the center 31 to the end portion32 according to the incident light dependency of the optical deflector12 for the light to be used.

Next, as illustrated in FIG. 6F, the photoresist pattern 63 was removedby the ultrasonic cleaning using a peeling liquid. At this time, thenickel alloy adhering to a sidewall of the photoresist pattern 63 duringthe ion milling was also removed. After the ultrasonic cleaning, thesurface of the nickel alloy pattern 64 was cleaned by electrolyticcleaning, and the microstructure mold 65 having an inverted structure ofthe microstructure formed in the scanning lens surface was obtained.

Next, as illustrated in FIG. 6G, the injection molding was performedusing the microstructure mold 65 and the mold including a stainless baseand a nickel alloy mirror surface and having no microstructure, so thatthe microstructure was transferred to the surface at the same time withformation of the scanning lens. The scanning lens 14 having themicrostructure in the surface was obtained in this manner. Regarding themicrostructure in the surface of the scanning lens 14 obtained in thefirst exemplary embodiment, three portions of the center 31, end portion32, and an intermediate portion between the center 31 and the endportion 32 were observed using an electron microscope. As a result, asubstantially inverted structure of the microstructure of themicrostructure mold 65 was obtained.

The second scanning lens 15 was obtained by injection molding using anordinary injection molding mold.

The scanning lens system 13 was configured using the produced scanninglenses 14 and 15, and the optical system illustrated in FIG. 1 wasproduced similarly to the scanning optical system for LBP products andevaluated. The evaluation results are illustrated in Table 1.

A second exemplary embodiment was similar to the first exemplaryembodiment except that a micro concavo-convex shape was formed in theoptical surfaces 14 a and 14 b of the first scanning lens 14 and in theoptical surfaces 15 a and 15 b of the second scanning lens, and thetransmittance distribution was formed in the optical surface 15 b. Thetransmittance distribution of the optical surface 15 b in the secondexemplary embodiment was made equal to the transmittance distribution ofthe optical surface 14 a in the first exemplary embodiment.

In the second exemplary embodiment, the first scanning lens 14 wasproduced similarly to the first exemplary embodiment except for thefollowing steps.

Steps in FIGS. 6A to 6C were performed similarly to the first exemplaryembodiment.

Next, in forming the nickel alloy pattern 64 by the plating method inFIG. 6D, the plating time was adjusted such that the height becomesapproximately 200 nm.

Next, in adjusting the height of the nickel alloy pattern 64 by the ionmilling method using Ar ions in FIG. 6E, the milling time of each areawas controlled such that the height of the center 31 becomesapproximately 181 nm, the height of the end portion 32 becomesapproximately 76 nm, and the height is monotonically lowered from thecenter 31 to the end portion 32.

In FIGS. 6F and 6G, the scanning lens 14 having the microstructure inthe surface was obtained, similarly to the first exemplary embodiment.

The evaluation results of the optical system of the second exemplaryembodiment are illustrated in Table 1.

A third exemplary embodiment was similar to the second exemplaryembodiment except that the transmittance distribution was formed in thetwo optical surfaces 15 a and 15 b of the second scanning lens 15.

In the third exemplary embodiment, the scanning optical lens 15 wasproduced similarly to the first exemplary embodiment except for thefollowing steps.

A method of forming the microstructure in the third exemplary embodimentwill be described with reference to FIGS. 7A to 7G. Note that FIGS. 7Ato 7G illustrate one surface in the process of forming themicrostructure.

As illustrated in FIG. 7A, an injection molding mold 71 for molding ascanning lens is prepared. The injection molding mold 71 including astainless steel base 71 a and a nickel alloy mirror surface 71 b isused.

As illustrated in FIG. 7B, a titanium film 72 and a silicon dioxide film73 are formed by a sputtering method. In the third exemplary embodiment,the titanium film 72 had the film thickness of approximately 50 nm, andthe silicon dioxide film 73 had the thickness of approximately 200 nm.

As illustrated in FIG. 7C, a photoresist layer 74 was formed by a spincoating method. Spin coating conditions in the third exemplaryembodiment were 3000 rpm/20 seconds, and the film thickness of thephotoresist layer 74 was approximately 150 nm.

Next, as illustrated in FIG. 7D, the photoresist layer 74 was exposed byan electron beam (EB) drawing method and was then developed, so that aphotoresist pattern 75 was obtained. In the third exemplary embodiment,a pillar pattern having a triangular lattice array was formed with acycle of 450 nm. Further, a pillar diameter was set to approximately 350nm at a center and approximately 401 nm at an end portion, and thepillar pattern was drawn such that the diameter is monotonically widenedfrom the center toward the end portion. The height was approximately 150nm, which is equivalent to the film thickness of the photoresist layer74.

Next, as illustrated in FIG. 7E, the silicon dioxide film 73 exposed ina recess of the photoresist pattern 75 was dry-etched by a dry etchingmethod using CHF3 gas, and a silicon dioxide pattern 76 was obtained. Inthe third exemplary embodiment, an etching time was controlled such thatthe height of the silicon dioxide pattern 76 becomes approximately 160nm.

Next, as illustrated in FIG. 7F, the photoresist pattern 75 was removedby an oxygen ashing method. Thereafter, a monomolecular release film(not illustrated) was formed on a surface of the silicon dioxide pattern76, so that a microstructure mold 77 was obtained.

Next, injection molding was conducted using a microstructure havingunevenness according to the microstructure to be formed in the othersurface of the scanning lens 15, which was produced by a similar methodto the microstructure mold 77, and the microstructure mold 77. By such amethod, the microstructure was transferred to the surface at the sametime with formation of the scanning lens, as illustrated in FIG. 7G. Thesecond scanning lens 15 having the microstructure in both the surfaceswas obtained in this manner Regarding the microstructure in the surfacesof the second scanning lens 15 obtained in the third exemplaryembodiment, three portions of the center, end portion, and anintermediate portion between the center and the end portion wereobserved using an electron microscope. As a result, a substantiallyinverted structure of the microstructure of the microstructure mold 77was obtained.

A fourth exemplary embodiment was similar to the first exemplaryembodiment except that the transmittance distribution was formed inthree surfaces of the optical surface 14 b of the first scanning lens 14and the optical surfaces 15 a and 15 b of the second scanning lens 15.The transmittance was monotonically changed between the center and theend portion in each of the optical surfaces 14 b, 15 a, and 15 b.

In the fourth exemplary embodiment, the first scanning lens 14 and thesecond scanning lens 15 were produced similarly to the third exemplaryembodiment except for the following steps.

Steps in FIGS. 7A to 7C were performed similarly to the third exemplaryembodiment.

Next, in forming a photoresist pattern 75 by an EB drawing methodillustrated in FIG. 7D, a pillar pattern having a triangular latticearray was formed with a cycle of 300 nm using a mold for 15 b surface.The pillar diameter was set to approximately 234 nm at a center andapproximately 202 nm at an end portion, and the pillar pattern was drawnsuch that the diameter is monotonically narrowed from the center towardthe end portion. A pillar pattern having a triangular lattice array wasformed with a cycle of 200 nm using a mold having an inverted structureof the optical surface 15 a and the optical surface 15 b. The pillardiameter was set to approximately 156 nm at the center and approximately119 nm at the end portion, and the pillar pattern was drawn such thatthe diameter is monotonically narrowed from the center toward the endportion. The depth was approximately 150 nm, which is equivalent to thefilm thickness of the photoresist layer 74.

Next, the scanning lenses 14 and 15 having the microstructure in thesurfaces were obtained similarly to the third exemplary embodiment inthe steps illustrated in FIGS. 6E to 6G.

A fifth exemplary embodiment was similar to the fourth exemplaryembodiment except for forming the transmittance distribution in the fouroptical surfaces 14 a, 14 b, 15 a, and 15 b. Regarding the transmittancedistribution, the transmittance at the center was 99.99% and thetransmittance at the end portion was 99.40%, and the transmittancebetween the center and the end portion was monotonically changed.

In the fifth exemplary embodiment, the first scanning lens 14 wasproduced similarly to the first exemplary embodiment except for thefollowing steps.

Steps in FIGS. 6A to 6C were performed similarly to the first exemplaryembodiment.

Next, in forming the nickel alloy pattern 64 by the plating method inFIG. 6D, the plating time was adjusted such that the height becomesapproximately 200 nm.

Next, in adjusting the height of the nickel alloy pattern 64 by the ionmilling method using Ar ions illustrated in FIG. 6E, the milling time ofeach area was controlled such that the height of the center becomesapproximately 160 nm, the height of the end portion becomesapproximately 122 nm, and the height is monotonically lowered from thecenter to the end portion.

In FIGS. 6F and 6G, the scanning lens 14 having the microstructure in asurface was obtained, similarly to the first exemplary embodiment.

TABLE 1 Transmittance Illuminance 14a 14b 15a 15b change uniformityFirst exemplary Transmittance Center 99.99% 96.00% 96.00% 96.00%Monotonically A embodiment End portion 97.60% 96.00% 96.00% 96.00%changes Reflectance Center 0.01% 4.00% 4.00% 4.00% End portion 2.40%4.00% 4.00% 4.00% Second exemplary Transmittance Center 99.99% 99.99%99.90% 99.99% Monotonically A embodiment End portion 99.99% 99.99%99.90% 97.60% changes Reflectance Center 0.01% 0.01% 0.10% 0.01% Endportion 0.01% 0.01% 0.10% 2.40% Third exemplary Transmittance Center99.99% 99.99% 99.99% 99.99% Monotonically A embodiment End portion99.99% 99.99% 98.80% 98.80% changes Reflectance Center 0.01% 0.01% 0.01%0.01% End portion 0.01% 0.01% 1.20% 1.20% Fourth exemplary TransmittanceCenter 99.99% 99.99% 99.99% 99.99% Monotonically A embodiment Endportion 99.99% 99.60% 99.00% 99.00% changes Reflectance Center 0.01%0.01% 0.01% 0.01% End portion 0.01% 0.40% 1.00% 1.00% Fifth exemplaryTransmittance Center 99.99% 99.99% 99.99% 99.99% Monotonically Aembodiment End portion 99.40% 99.40% 99.40% 99.40% changes ReflectanceCenter 0.01% 0.01% 0.01% 0.01% End portion 0.60% 0.60% 0.60% 0.60%

(Evaluation)

In the first exemplary embodiment, the microstructure having thecylindrical hole was provided in the optical surface 14 a of the firstscanning lens 14. By forming the holes such that the depth of the holesincreases from the center toward the end portion and the transmittanceat the lens center was 99.99% and the transmittance at the lens endportion was 97.60%. As a result, the transmittance monotonically changedbetween the lens center and the lens end portion. The light quantitydistribution on the surface to be scanned 16 was able to be uniformizedMeanwhile, the reflectance at the center of the optical surface 14 a ofthe first scanning lens 14 was 1% or less, but the reflectance of theother optical surfaces was higher than 1%.

Although stray light occurs due to reflection in a region where thereflectance exceeds 1%, the reflectance at the center of the opticalsurface 14 a, which is most likely to reach the surface to be scanned16, is 1% or less, so the sufficiently uniform light quantitydistribution was able to be realized on the surface to be scanned 16.

In the second exemplary embodiment, the light quantity distribution onthe surface to be scanned 16 was able to be uniformized Moreover, thereflection on the optical surfaces 14 a and 14 b of the first scanninglens 14 and the optical surfaces 15 a and 15 b of the second scanninglens 15 of the scanning lens system was suppressed to increase thetransmittance. Therefore, a loss of the light beam was able to bereduced. The reduction of the loss of the light beam leads to reductionof the cost of the light source or improves a scanning rate. Thereflectance at the end portion of the optical surface 15 b of the secondscanning lens 15 was higher than 1% but the reflectance at the otheroptical surfaces was 1% or less. Therefore, it was confirmed that asufficiently uniform light quantity distribution can be obtained on thesurface to be scanned 16.

In the third exemplary embodiment, the light quantity distribution onthe surface to be scanned 16 was able to be uniformized similarly to thesecond exemplary embodiment, and the transmittance of all the surfacesof the scanning lens system was increased. Therefore, the loss of thelight beam was able to be reduced. Moreover, the transmittancedistribution was provided using the two scanning lens surfaces. As aresult, the transmittance distribution per surface was able to bereduced. The reflectance at the end portions of the optical surfaces 15a and 15 b was able to be suppressed as compared with the reflectance atthe end portion of the optical surface 15 b of the second exemplaryembodiment, but the reflectance was not able to be reduced to 1% orless. However, since the reflectance at the centers of all the opticalsurfaces was 1% or less, a sufficiently uniform light quantitydistribution was able to be realized on the surface to be scanned 16.

In the fourth exemplary embodiment, the light quantity distribution onthe surface to be scanned 16 was able to be uniformized similarly to thethird exemplary embodiment, and the loss of the light beam was able tobe reduced. Moreover, since the transmittance distribution was providedusing three scanning lens surfaces, the transmittance distribution persurface was able to be reduced, the reflectance of all the opticalsurfaces was able to be 1% or less, the stray light was reduced ascompared with the first to third exemplary embodiments, and a moreuniform light quantity distribution was able to be obtained on thesurface to be scanned 16.

In the fifth exemplary embodiment, the light quantity distribution onthe surface to be scanned 16 was able to be uniformized similarly to thefourth exemplary embodiment, and the loss of the light beam was able tobe reduced. Further, the reflectance of all the optical surfaces wasable to be reduced to 1% or less. Similarly to the fourth exemplaryembodiment, the stray light was reduced, and the uniform light quantitydistribution was able to be obtained on the surface to be scanned 16.

From the results of the first to fifth exemplary embodiments, thedifference in the transmittance between the lens center and the lens endportion was able to be made less than 1.9%. Further, regarding the firstscanning lens 14 and the second scanning lens 15, the reflectance fromthe center to the end portion was set to 1.0% or less, whereby a moreuniform light quantity distribution was able to be obtained on thesurface to be scanned 16 than that of other exemplary embodiments.

While the disclosure has been described with reference to exemplaryembodiments, it is to be understood that the disclosure is not limitedto the disclosed exemplary embodiments. The scope of the followingclaims is to be accorded the broadest interpretation so as to encompassall such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Applications No.2019-187964, filed Oct. 11, 2019, and No. 2020-153004, filed Sep. 11,2020, which are hereby incorporated by reference herein in theirentirety.

What is claimed is:
 1. A system comprising: a light source; a deflectionunit configured to deflect a light beam having a wavelength λ emittedfrom the light source; and a lens unit including a plurality of lensesthat focuses deflected light on a surface to be scanned, wherein atleast one lens among the plurality of lenses has a micro concavo-convexstructure in an optical surface, and wherein the optical surface has atransmittance distribution for the light beam having the wavelength λaccording to a light quantity distribution of the deflected light andentering the lens unit.
 2. The system according to claim 1, wherein thelight beam having passed through the lens unit has a smaller opticaldistribution than the light beam before passing through the lens unit.3. The system according to claim 1, wherein a lens surface having themicro concavo-convex structure in the surface has a larger transmittanceat a lens center than a transmittance at a lens end portion.
 4. Thesystem according to claim 3, wherein, in the lens surface, thetransmittance of the light beam having the wavelength λ monotonicallydecreases from the lens center toward the lens end portion.
 5. Thesystem according to claim 4, wherein, in the micro concavo-convexstructure in the lens surface, a difference in heights of a top of aconvex and a bottom of a concave monotonically decreases from the lenscenter toward the lens end portion in a scanning direction.
 6. Thesystem according to claim 1, wherein the micro concavo-convex structurein the lens surface is a hole structure.
 7. The system according toclaim 1, wherein two or more optical surfaces of the plurality of lenseshave the micro concavo-convex structure.
 8. The system according toclaim 7, further comprising: a first lens and a second lens, wherein thefirst lens or the second lens has the micro concavo-convex structure inoptical surfaces opposite each other.
 9. The system according to claim8, wherein each of the first lens and the second lens has the microconcavo-convex structure in both of the optical surfaces opposite eachother.
 10. An apparatus comprising: a scanning device including asystem, wherein the system comprises: a light source; a deflection unitconfigured to deflect a light beam having a wavelength λ emitted fromthe light source; and a lens unit including a plurality of lenses thatfocuses deflected light on a surface to be scanned, wherein at least onelens among the plurality of lenses has a micro concavo-convex structurein an optical surface, and wherein the optical surface has atransmittance distribution for the light beam having the wavelength λaccording to a light quantity distribution of the deflected light andentering the lens unit; a photosensitive drum disposed on a surface tobe scanned of the optical scanning device; a developing unit configuredto develop, as a toner image, an electrostatic latent image formed by alight beam scanning the photosensitive drum; a transfer unit configuredto transfer the developed toner image on a sheet; and a fixing unitconfigured to fix the transferred toner image on the sheet.
 11. Theapparatus according to claim 10, wherein the light beam having passedthrough the lens unit has a smaller optical distribution than the lightbeam before passing through the lens unit.
 12. The apparatus accordingto claim 10, wherein a lens surface having the micro concavo-convexstructure in the surface has a larger transmittance at a lens centerthan a transmittance at a lens end portion.
 13. The apparatus accordingto claim 12, wherein, in the lens surface, the transmittance of thelight beam having the wavelength λ monotonically decreases from the lenscenter toward the lens end portion.
 14. The apparatus according to claim13, wherein, in the micro concavo-convex structure in the lens surface,a difference in heights of a top of a convex and a bottom of a concavemonotonically decreases from the lens center toward the lens end portionin a scanning direction.
 15. The apparatus according to claim 10,wherein the micro concavo-convex structure in the lens surface is a holestructure.
 16. The apparatus according to claim 10, wherein two or moreoptical surfaces of the plurality of lenses have the microconcavo-convex structure.
 17. The apparatus according to claim 16,wherein the system further comprises: a first lens and a second lens,wherein the first lens or the second lens has the micro concavo-convexstructure in optical surfaces opposite each other.
 18. The apparatusaccording to claim 17, wherein each of the first lens and the secondlens has the micro concavo-convex structure in both of the opticalsurfaces opposite each other.